Why is my didBeginContact function detecting way too many collisions?

Question

My didBeginContact function is detecting about 60 collisions between two SKSpriteNodes when there should only be one.

func didBeginContact(contact: SKPhysicsContact) {
    let firstBody = contact.bodyA
    let secondBody = contact.bodyB
    if firstBody.categoryBitMask == bulletCategory && secondBody.categoryBitMask == dragonCategory || firstBody.categoryBitMask == dragonCategory && secondBody.categoryBitMask == bulletCategory{

        print("collision happened")
    }
}

When the nodes come into contact the console prints "collision happened" many many times.

Here is how i declare physics bodys for each SKSpriteNode

dragonNode.physicsBody = SKPhysicsBody(texture: dragonNode.texture!, size: CGSizeMake(dragonNode.size.width, dragonNode.size.height))
dragonNode.physicsBody?.affectedByGravity = false              
dragonNode.physicsBody?.dynamic = true
dragonNode.physicsBody?.categoryBitMask = dragonCategory
dragonNode.physicsBody?.collisionBitMask = bulletCategory
dragonNode.physicsBody?.contactTestBitMask = bulletCategory

bulletNode.physicsBody = SKPhysicsBody(texture: texture, size: texture.size())
bulletNode.physicsBody?.affectedByGravity = false
bulletNode.physicsBody?.dynamic = true
bulletNode.physicsBody?.categoryBitMask = bulletCategory
bulletNode.physicsBody?.collisionBitMask = dragonCategory
bulletNode.physicsBody?.contactTestBitMask = dragonCategory

This is one of the last things i need to fix for my game to be completed so if anyone can help that would be great!

Answer 1

It's because theoretically your game is running at 60 frames per second. So every 60th of a second it is detecting the collision and printing the warning. You haven't put any code into say stop detecting this collision. You could always put a flag on the object

dragonNode.collided = true

and then detect for the collision and the flag

if (firstBody.categoryBitMask == bulletCategory && secondBody.categoryBitMask == dragonCategory || firstBody.categoryBitMask == dragonCategory && secondBody.categoryBitMask == bulletCategory) && dragonNode.collided != true {
    dragonNode.collided = true
    print("collision happened")
}

this way the collision only fires once